How to use voltage map of BFLEX25 main board

KLD BFLEX25 is Bass amp based on Ampeg PF20T. The power amp is Class AB amp of two 6L6 tubes. The voltage map of main board of each tube guitar amps, could help hobbyists to understand tube amp principle and find assembly questions.

Please pay attention: The inner of tube guitar amp has fatal high voltage. If you have not enough skill and experience in high voltage circuit, Do not try to test it .

Measure condition of voltage map
The voltage map of BFLEX25 main board is measured in these status. Please adjust your amp to these status.There are the differents between the electronic components, so it is normal that 10% fluctuation of voltage value between your amp with voltage map.
Voltage: DC
Standby ON
Contol: Gain and Master minimum
Biase voltage: -30V
Power tube: Pentode

BBFLEX25 voltage map of main board

Voltage map of BFLEX25 main board
Start from down to up of board. The power supply filter circuit is on the downest of the board. The rightest is the rectifier bridge consisted by four diodes D1-D4, which provides full-wave rectify for 260 VAC from power transformer. Then through different capacitors filting , we get different plate voltages of the tube in amp stages: B+A: 390VDC, B+B: 360VDC, B+C: 308VDC, B+D:270VDC (V2). B+E:255VDC (V2).
If you could not hear any sound opening amp , please check these voltage normal first, then check the points connected with them in tube sockets.The anodes of 12ax7 are Pin1, 6, the anode of 6L6 is Pin3.
Then, the uppest line of the main board.Each one has the position code of one letter and two digits. The first letter “V” represent "Valve (Tube) ". First digital means code of tubes, Second digital means code of pin of tube. For example : V2-3 reprent this point will be connected with Pin3 of second vacuum tube.
Because this main board is universal board of our amps, so the position code printed on board are not whole consistent with real layout drawing of each amp. Please connect wires according to layout drawing of each amp kits.
BFLEX25 only has two amplify stages, V1B, V1A. The voltages of V1 are below: The anode voltage of V1B is 149VDC(V1-6 of board), The anode voltage of V1B is 144VDC(V1-1 of board), and the cathode voltage of V1B is 1.6VDC (V1-8) and V1A is 1.3VDC (V2-3).

BFLEX25 layout

The second 12ax7 is Long Tail Pair(LTP) phase inverter of BFLEX25 amp.The grids (Pin2,7) of V2 connected with SKY and V2-8 of the board, the voltages of these two points are 0VDC. The anodes( Pin1,6) of V2 connected with V3-1 and V3-6 of the board.The voltage of V3-1 is 211VDC and V3-6 is 190VDC. The cathodes of V2 are Pin 3 and 8 of tube, they connected with one point of the board V3-3/8, the voltage is 1.6VDC.

The power tube of BFLEX25 can use 6L6, EL34 etc octal pins power tubes. We used 6l6 being power tube in the test. Pin2 and 7 are pins of filament. Pin3 is Anode(Plate) . The pin 4 is grid2(Screen). Pin5 is grid1.Pin8 is cathode and beam plate. The power amp of BFLEX25 is class AB : V3 and V4. The pins of these two 6L6 connected with the main board include : Pin2s(Filament)of of V3 and V4 connected with board 6.3VAC; Pin4s(Grid 2) of V3 and V4 connected with V4-9,V5-9 of the board, the voltages are 330VDC all; Pin5s(Grid 1) connected with V5-2 and V4-2 of the board, one is -43VDC, one is -40VDC; Pin8s (Cathode) connected with V4/5-3 and R24C of the board, all are 80 mVDC.